The characteristic of the angular modulation curves of the reflectance ratio γ for p-polarized beams from both sides of coated plane glass is closely related to the optical parameters of the films. The dependence of relative gradients of γ to the refractive index n, extinction coefficient k and thickness h of the films on the optical parameters are obtained theoretically, which provides theoretical basis for the optimization of parameters. By data simulation, the resolution of refractive index of film sensor based on the calculation is predicted to be less than 10-7. It has been measured that the optical parameters of sol-gel SnO2 films under different technological conditions and the sensitivity to the testing gas. The results indicate that the method of the p-polarized reflectance can be widely used in high sensitivity optical chemical sensors.